Sustained vs. Unsustained Clonus: Clinical Definitions
Unsustained clonus consists of a few beats (typically 2-10 rhythmic contractions) that spontaneously cease, while sustained clonus persists continuously for more than 10 beats or indefinitely until the stimulus is removed. 1
Key Distinguishing Features
Unsustained Clonus
- Limited duration: Consists of only a few rhythmic muscle contractions (commonly 2-10 beats) before spontaneously stopping 1
- Clinical significance: Often represents a borderline upper motor neuron sign that may be seen in normal individuals or mild neurological conditions 1
- Example presentation: A patient with remote spinal cord injury showing "2 beats of unsustained clonus in each ankle" with otherwise full sensorimotor function represents a mild residual finding 1
Sustained Clonus
- Prolonged duration: Rhythmic contractions continue indefinitely as long as the stretch stimulus is maintained, or persist for more than 10 beats 2
- Pathological significance: Indicates definite upper motor neuron dysfunction from conditions such as stroke, spinal cord injury, multiple sclerosis, or severe metabolic derangements (hepatic failure, serotonin syndrome) 2
- Mechanism: Results from a combination of increased reflex gain (reduced motor neuron threshold), enlarged tissue viscoelasticity, and long reflex pathway delays, not solely from recurrent muscle stretch 3, 4, 5
Clinical Testing Technique
To elicit clonus: Apply rapid dorsiflexion of the ankle (or other joint) and maintain steady stretch pressure 2, 6
- Unsustained response: A few beats occur then stop despite maintained stretch
- Sustained response: Rhythmic oscillations continue as long as stretch is maintained 2, 6
Important Clinical Distinctions
Clonus vs. Myoclonus
Do not confuse clonus with myoclonus—these are fundamentally different phenomena:
- Clonus: Rhythmic, predictable oscillations triggered and maintained by sustained muscle stretch, involving alternating contraction-relaxation cycles 2, 3
- Myoclonus: Sudden, brief, lightning-like jerks that are non-rhythmic and do not require sustained stretch to occur 7, 8
Prognostic Implications
- Unsustained clonus (few beats): Compatible with good functional recovery and may persist as a benign residual finding years after neurological injury 1
- Sustained clonus: Indicates more severe upper motor neuron dysfunction requiring ongoing management, though it does not necessarily predict poor functional outcome in chronic conditions 2, 6
Common Pitfall to Avoid
The critical error is counting beats incorrectly or releasing stretch pressure prematurely. Maintain consistent dorsiflexion pressure throughout testing—if oscillations stop while you're still applying stretch, this is unsustained clonus; if they continue indefinitely requiring you to release the stretch to stop them, this is sustained clonus 1, 2.